全文获取类型
收费全文 | 9001篇 |
免费 | 997篇 |
国内免费 | 1篇 |
专业分类
9999篇 |
出版年
2021年 | 105篇 |
2019年 | 71篇 |
2018年 | 117篇 |
2017年 | 111篇 |
2016年 | 150篇 |
2015年 | 277篇 |
2014年 | 300篇 |
2013年 | 400篇 |
2012年 | 450篇 |
2011年 | 452篇 |
2010年 | 295篇 |
2009年 | 238篇 |
2008年 | 395篇 |
2007年 | 442篇 |
2006年 | 382篇 |
2005年 | 392篇 |
2004年 | 344篇 |
2003年 | 328篇 |
2002年 | 312篇 |
2001年 | 187篇 |
2000年 | 196篇 |
1999年 | 165篇 |
1998年 | 134篇 |
1997年 | 134篇 |
1996年 | 120篇 |
1995年 | 103篇 |
1994年 | 124篇 |
1993年 | 107篇 |
1992年 | 176篇 |
1991年 | 137篇 |
1990年 | 124篇 |
1989年 | 128篇 |
1988年 | 119篇 |
1987年 | 135篇 |
1986年 | 107篇 |
1985年 | 148篇 |
1984年 | 155篇 |
1983年 | 111篇 |
1982年 | 132篇 |
1981年 | 119篇 |
1980年 | 109篇 |
1979年 | 102篇 |
1978年 | 75篇 |
1977年 | 75篇 |
1976年 | 85篇 |
1975年 | 66篇 |
1974年 | 84篇 |
1973年 | 74篇 |
1972年 | 63篇 |
1971年 | 61篇 |
排序方式: 共有9999条查询结果,搜索用时 0 毫秒
31.
32.
33.
34.
Abstract The effects of an unusual high frequency mowing regime, which involved the removal of slash, were compared to moderate grazing through the method of paired quadrats across a fenceline, which was orthogonal to a weak environmental gradient. The mown plots proved superior in their conservation characteristics to the moderately grazed plots. The mowing regime produced greater cover of rare or threatened species, greater native cover and lesser exotic grass cover. It thus presents an opportunity for maintaining or improving the condition of previously grazed remnants in reserves without resorting to the use of stock or fire for biomass reduction. 相似文献
35.
36.
The gamma aminobutyric acid (GABA) antagonists picrotoxin and bicuculline were injected into the striatum of rats via chronic cannulae. Dyskinesias were produced by these drugs which could be blocked by their injection combined with GABA. The intrastriatal (i.s.) injection of a cholinergic drug, carbachol, also produced dyskinesias which were blocked by GABA. The use of i.s. injection of GABA antagonists to produce an animal model of Huntington's chorea is discussed. 相似文献
37.
38.
Hanafi H. Russell Richard J. Jackson David P. Spath Steven A. Book 《The Western journal of medicine》1987,147(5):615-622
Drinking water contamination by toxic chemicals has become widely recognized as a public health concern since the discovery of 1,2-dibromo-3-chloropropane in California''s Central Valley in 1979. Increased monitoring since then has shown that other pesticides and industrial chemicals are present in drinking water. Contaminants of drinking water also include naturally occurring substances such as asbestos and even the by-products of water chlorination. Public water systems, commercially bottled and vended water and mineral water are regulated, and California is also taking measures to prevent water pollution by chemicals through various new laws and programs. 相似文献
39.
G Chen H J Strobel J B Russell C J Sniffen 《Applied and environmental microbiology》1987,53(9):2021-2025
When mixed ruminal bacteria were incubated with a pancreatic casein hydrolysate and free amino acids of a similar composition, rates of ammonia production were much greater for peptides than for amino acids. The pancreatic digest of casein was then fractionated with 90% isopropyl alcohol. Hydrophobic peptides which dissolved in alcohol contained an abundance of phenolic and aliphatic amino acids, while the hydrophilic peptides which were precipitated by alcohol contained a large proportion of the highly charged amino acids. The Km values of the mixed ruminal bacteria for each fraction were similar (0.88 versus 0.98 g/liter), but the Vmax of the hydrophilic peptides was more than twice that of the hydrophobic peptides (18 versus 39 mg of NH3 per g of bacterial protein per h). Pure cultures of ruminal bacteria had a similar preference for hydrophilic peptides and likewise utilized peptides at a faster rate than free amino acids. Since peptide degradation rates differed greatly, hydrophobicity is likely to influence the composition of amino acids passing unfermented to the lower gut of ruminant animals. 相似文献
40.
Bacteroides ruminicola B(1)4, a predominant ruminal and cecal bacterium, was grown in batch and continuous cultures, and beta-glucosidase activity was measured by following the hydrolysis of p-nitrophenyl-beta-glucopyranoside. Specific activity was high when the bacterium was grown in batch cultures containing cellobiose, mannose, or lactose (greater than 286 U/g of protein). Activity was reduced approximately 90% when the organism was grown on glucose, sucrose, fructose, maltose, or arabinose. The specific activity of cells fermenting glucose was initially low but increased as glucose was depleted. When glucose was added to cultures growing on cellobiose, beta-glucosidase synthesis ceased immediately. Catabolite repression by glucose was not accompanied by diauxic growth and was not relieved by cyclic AMP. Since glucose-grown cultures eventually exhibited high beta-glucosidase activity, cellobiose was not needed as an inducer. Catabolite repression explained beta-glucosidase activity of batch cultures and high-dilution-rate chemostats where glucose accumulated, but it could not account for activity at slow dilution rates. Maximal beta-glucosidase activity was observed at a dilution rate of approximately 0.35 h-1, and cellobiose-limited chemostats showed a 15-fold decrease in activity as the dilution rate declined. An eightfold decline was observed in glucose-limited chemostats. Since inducer availability was not a confounding factor in glucose-limited chemostats, the growth rate-dependent derepression could not be explained by other mechanisms. 相似文献